The generation of ion or electron optical imaging systems which have been corrected with respect to their chromatic aberration are complicated when compared to light optics. This situation is present because the electric and/or magnetic round lenses used for imaging always have positive chromatic aberrations. A correction of such imaging systems by means of a plurality of lenses arranged one behind the other is therefore not possible.
From the article of H. Rose entitled "Outline of a spherically corrected semiaplanatic medium-voltage transmission electron microscope" published in Optik, Volume 85, No. 1 (1990), page 19, it is known that spherical aberrations of an electron optic system can be corrected by two magnetic doublet systems and a magnetic hexapole element downstream of each doublet. It is also possible to obtain a simultaneous correction of spherical and chromatic aberrations by means of octupoles and combined electric-magnetic quadrupole elements; however, very high requirements are then imposed on the stability of the quadrupole fields as well as on the adjustment of the multipoles.
Furthermore, it is known to simultaneously correct chromatic and spherical aberrations by utilizing electrostatic mirrors since chromatic and spherical aberrations of electrostatic mirrors can be negative. An appropriate imaging system is shown, for example, in the article of G. F. Rempfer entitled "A theoretical study of the hyperbolic electron mirror as a correcting element for spherical and chromatic aberration in electron optics" published in the Journal of Applied Physics, Volume 67, No. 10, May 15, 1990, page 6027. For a mirror correction arrangement of this kind, a beam deflector is required which deflects the electron beam coming from the objective to the mirror and which deflects the electron beam reflected at the mirror into the viewing beam path. Magnetic beam deflectors, as a rule, themselves cause aberrations of the second order since they are not rotationally symmetric. To prevent these additional aberrations, the suggestion is made in the last-mentioned article that the deflection take place in intermediate image planes. Overall, a system is needed here which comprises three deflectors with interposed relay lens systems for each deflector so that the entire imaging system is very complex and costly.
A mirror correction arrangement is disclosed in the article of V. Kolarik et al entitled "Close packed prism arrays for electron microscopy" published in Optik, Volume 87, No. 1, (1991), page 1. In this mirror correction arrangement, a single 90.degree. deflector having five sectors of different magnetic field strength is provided. The symmetry planes of the deflector are at the same time also intermediate image planes. The mirror itself is mounted in the image plane of a second cathode lens which is mounted rearward of a diffraction plane disposed outside of the deflector. The aberrations of the second order of the deflector here remain after a two-time passthrough of the electronic beam through the deflector.